A photo-sulfoxidation method is known in which a saturated hydrocarbon is photo-sulfoxidized with sulfur dioxide and oxygen in a substantially water-free reaction system. In this method, colored materials are deposited on the surface of a light source and will impair the light illumination to such an extent that the intended photo-sulfoxidation is considerably retarded. When certain amounts of such colored materials are deposited on the surface of the light source, the reactor must be shut down for the purpose of removing the deposit, but this is quite inefficient and makes continuous operation of the reactor impossible.
Under these circumstances, the commercial production of alkanesulfonic acids has been limited to photo-sulfoxidation in the presence of water. However, the reaction efficiency of this method is low because the water used inhibits the radical chain reaction. In addition, sulfuric acid is formed as a by-product in an amount substantially equimolar to the intended alkanesulfonic acid, and in order to separate the sulfuric acid, the water must be distilled off at from 90.degree. to 140.degree. C. This produces an undesired odor or color in the alkanesulfonic acid.
Even the method of photo-sulfoxidation in the presence of water is not completely free from the formation of colored materials. A method has been proposed for inhibiting the formation of such colored materials, which involves first irradiating the saturated hydrocarbon with UV light in the presence of sulfur dioxide, but in the substantial absence of oxygen, and then introducing oxygen into the irradiated saturated hydrocarbon (Japanese Patent Publication No. 28973/1972). It has also been proposed that suppression of the deposition of colored materials on the surface of a light source may be accomplished by causing a predetermined amount of nitrogen or air to flow in the area near the surface of the light source (Japanese Patent Publication No. 11740/1972).
In the first method wherein the saturated hydrocarbon is illuminated with UV light in the substantial absence of oxygen, the inadequate presence of oxygen radicals, which are an important driving force for the photo-sulfoxidation, causes the reaction to proceed at a very slow rate and makes this method unsuitable for commercial operation. In the second method wherein a stream of nitrogen or air is caused to flow in the area near the surface of the light source, contact between said surface and the reaction mixture cannot be completely prevented, and the eventual resulting fouling of the surface of the light source is still inevitable. In addition, the flowing stream of nitrogen or air drives sulfur dioxide out of the reaction mixture, so the concentration of sulfur dioxide in the reaction mixture is reduced, and thus slows down the reaction rate.
On the other hand, no proposal has been made which provides for effectively inhibiting the formation of colored materials and preventing the deposition of such materials on the surface of a light source used in the photo-sulfoxidation of saturated hydrocarbons in the substantial absence of water.